Electromagnetic energy dosimeter

ABSTRACT

Miniaturized dosimeter apparatus monitors the field density of radiated pulsed electromagnetic energy. An elongate cylindrical coil is used as a pickup and is positioned so that one-half of the coil protrudes from a small metallic case for the apparatus, providing uniform response substantially over a spherical angle of at least 180*. Means is interconnected with the coil for producing a periodic waveform having an average magnitude varying as a function of the field density of the radiated energy and having a period inversely proportional to the pulse repetition frequency of the energy. Means is responsive to this periodic waveform for providing a readily perceptible indication of the average magnitude of the periodic waveform whereby the field density of the radiated energy, and thereby the relative human thermogenic dosage thereof, is readily monitored.

nited States Patent v Richardson [.45] Feb. 1, 1972 54] ELECTROMAGNETICENERGY DOSIMETER [21] Appl. No.: 3,366

U.S. Cl..' ..325/363, 250/39, 325/364 Int. Cl. Field of Search ..325/67,149, 363, 364;

[56] References Cited UNITED STATES PATENTS 5 PUBLICATIONS ElectronicsWorld; June 1961, pp. 31- 33 & 79; Detecting Microwave-RadiationHazards," by Tom Jaski.

Primary Examiner-Benedict V. Safourek Attorney-Koonig, Senniger, Powersand Leavitt [57] ABSTRACT Miniaturized dosimeter apparatus monitors thefield density of radiated pulsed electromagnetic energy. An elongatecylindrical coil is used as a pickup and is positioned so that one-halfof the coil protrudes from a small metallic case for the apparatus,providing uniform response substantially over a spherical angle of atleast 180. Means is interconnected with the coil for producing aperiodic waveform having an average magnitude varying as a function ofthe field density of the radiated energy and having a period inverselyproportional to the pulse repetition frequency of the energy. Means isresponsive to this periodic waveform for providing a readily perceptibleindication of the average magnitude of the periodic waveform whereby thefield density of the radiated energy, and thereby the relative humanthermogenic dosage thereof, is readily monitored.

11 Claims, 3 Drawing Figures ELECTROMAGNETIC ENERGY DOSIMETER BACKGROUNDOF THE INVENTION This invention relates to apparatus for monitoring thefield density of radiated pulsed electromagnetic energy and moreparticularly to such apparatus for providing readily perceptible visualor auditory indication of the field density of such radiated energy formonitoring the relative human thermogenic dosage of such energy.

Many situations occur where it is desired to know if human exposure toradiated electromagnetic radio frequency energy is dangerous. Many kindsof apparatus, such as certain types of radar, other microwave devices,including microwave ovens, and radio frequency transmitters radiateelectromagnetic energy which has field density of sufficient magnitudethat it is necessary to guard against human exposure to such radiationin order to insure that biological damage will not result fromthermogenic heating within the human tissue and organs exposed thereto.While the extent of such heating is a function of both the field densityof such radiated energy and the time of exposure thereto, a measure ofthe field density of such energy provides a relative indication of thehuman thermogenic dosage resulting from such exposure. For example, amaximum field density of radiated energy which is safe for humanexposure has been established by the armed forces of the United Statesto be milliwatts per square centimeter (mW./cm.=), a figure which hasbeen accepted by segments of both government and industry. Accordingly,a dosimetric indication of exposure to such microwave or other radiofrequency energy may be given by apparatus which detects the fielddensity of such energy and provides an indication of the field densitywith respect to such a predetermined maximum safe,

field density. Conventional radio field strength measuring apparatus isnot useful for monitoring the overall field density of radiatedelectromagnetic energy to which there will be human exposure, since suchapparatus is generally highly directional in nature and responsive onlyto a certain frequency or, at best, distinct bands of frequencies.Dosimeters have been proposed which simulate the actual heating effect,i.e., the thermogenic effect, on human tissue by actually measuring thetemperature of a mass which behaves analogously to human tissue. Whileproviding a cumulative indication of dosage, such apparatus isundesirably slow to respond and is therefore not quickly indicative ofhigh field density levels of radiated energy. Other proposed types ofdosimeters measure the actual field density of radiated energy, but havegenerally been found to be undesirable because of inaccuracy,sensitivity only to limited frequencies, or because their pickuparrangement is directional in character or has nulls or other nonuniformreceptive qualities. Such devices must therefore be aimed or carefullyaligned with respect to the source of the energy. The latterdisadvantage particularly greatly detracts from the practicality andusefulness of such devices. The resultant lack of confidence in theindicated radiation level further compromises the use of such prior artdevices. Another disadvantage of dosimeters of the prior art has beentheir lack of suitability for being conveniently carried by a personwithout interfering with his other duties. For example, such deviceshave not been suited for being carried in a pocket or on one 's belt.Disadvantageously, these prior art devices have typically beenrelatively large, complicated, and accordingly, expensive.

SUMMARY OF THE INVENTION Among the several objects of the invention maybe noted the provision of dosimeter apparatus for monitoring the fielddensity of radiated pulsed electromagnetic energy; the provision of suchapparatus which provides a readily perceptible indication of the fielddensity of such energy with respect to a predetermined maximum fielddensity which is safe for human exposure; the provision of suchapparatus which is uniformly responsive to radiated energy substantiallyover a spherical angle of greater than 180 thereby being essentiallynondirectional in use; the provision of such apparatus which is highlysensitive to radiated radio frequency energy over a ivide frequencyspectrum; the provision of such apparatus which is extremely small insize, reliable in operation, relatively simple, and inexpensivelyconstructed. Other objects and features will bein part apparent and inpart pointed out hereinafter.

Briefly, dosimeter apparatus of the present invention monitors the fielddensity of radiated pulsed electromagnetic energy. The apparatus isprovided with a pickup which is responsive to the radiated energy andwhich is constituted by an elongate cylindrical coil having a pluralityof turns of wire spaced uniformly along the length thereof. Means isprovided, preferably a metallic closure for the apparatus, forelectromagnetically shielding half the length of the coil beginning atone'end thereof. Thus the radiation pattern of the coil is such that thecoil is uniformly responsive to the radiated energy substantially over aspherical angle of at least 180, i.e., 360 ina horizontal plane and atleast l80 in a vertical plane. Means is interconnected with the coil forproducing a periodic waveform having an average magnitude which variesas a function of the field density of the radiated energy and having aperiod which is inversely proportional to the pulse repetition frequencythereof. Finally, means is responsive to this periodic waveform forproviding a readily perceptible indication of the average magnitude ofthe periodic waveform. In this way, the field density of said radiatedenergy, and thereby the relative human thermogenic dosage thereof, isreadily monitored.

,BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic circuit diagramof dosimeter apparatus of the present invention;

FIG. 2 is a view in perspective of a miniaturized dosimeter of thisinvention; and

FIG..'3 is a section, taken along line 33 of FIG. 2, of a pickupassembly of the dosimeter of FIG. 2 also representing graphically theradiation pattern of the pickup.

Corresponding reference characters indicate corresponding partsthroughout the several views of the drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings,and more particularly to FIG. 1, a schematic circuit diagram illustratescircuitry of dosimeter apparatus of the present invention which isadapted to monitor the field density of radiated pulsed electromagneticenergy,i.e., radiofrequency energy which is transmitted with a pulsedcharacteristic and which therefore has a pulse repetition frequency.Such energy is radiated, for example, in the operation of radar systems.

The apparatus includes a pickup coil 11, described in greater detailhereinbelow, which is responsive to the radiated energ'yzOne side ofcoil 11 is connected to a circuit ground or common COM. A voltagedivider comprising three series-connected resistors R1, R2 and R3 isconnected across coil ll, one side of each of the resistors beingconnected to a respective tsp of a selector switch SW1 permittingselection of three ranges of sensitivity of the apparatus. The inputsignal is provided through a capacitor C1 to a notch filter indicatedgenerally at 13. Capacitor Cl, it should be understood, is relativelylarge with respect to the frequencies picked up by coil 11 and is thusnot employed in any sense for tuning purposes but rather for isolatingcoil 11 from a DC biasing voltage described hereinafter and to preventany coupling of low frequencies from coil 11 into the apparatus becauseof the time constant it provides. Thus coil 11 may be said to beconnected in an untuned circuit.

Notch filter 13, which is optional, is of a parallel-T configurationwith a first T including a pair of resistors R4 and R5 and a capacitorC2 and a second T having a pair of capacitors C3 and C4 and a resistorR6. Notch filter 13 is adapted to shunt any signal picked up at apreselected notch rejection frequency determined by the value of thecomponents of the notch filter. This preselected frequency may, forexample, be 60 Hz.

or 400 Hz, these frequencies being characteristic of conventional ACpower sources used for powering apparatus whose transmitted output isbeing monitored by a dosimeter of this invention. For example, 400 Hz.power is used for powering military radars and the various powerconnections incidental to such equipment are likely to generateelectromagnetic fields at this frequency. It is desired that the presentdosimeter not be responsive, therefore, to such a power frequency.

The input signal passed through notch filter 13 is rectified by a diodeD1 and applied across a capacitor C5, one side of the latter beingconnected to the circuit common COM. The rectified signal acrosscapacitor C5 is supplied to the noninverting input, indicated by a plussign, of a differential amplifier A and also to the similarly indicatednoninverting input of another differential amplifier 158. Each ofamplifiers 15A and 15B is preferably a monolithic operational amplifierof the linear integrated circuit variety available from a number ofsources. Such a device, by virtue of its extremely small size,contributes to miniaturization of the present dosimeter apparatus. Inorder to insure accuracy and overall satisfactory performance, it ispreferred that each of amplifiers 15A and 153 exhibits relatively goodperformance. For example, the amplifiers should have good linearity toinsure accuracy and should have relatively high open-loop gain to insuresensitivity. In a practical embodiment of this dosimeter, it was foundthat satisfactory performance was obtained by using for each ofamplifiers 15A and 158 an RCA-CA302O integrated circuit combinedpreamplifier-driver.

The inverting input, designated with a minus sign, of each of amplifiers15A, 15B is connected to the circuit common COM through a respectivecapacitor C6A, C68 to provide each of the amplifiers with temperaturestabilization and current compensation. A gain control for each of theamplifiers is provided by means of a respective potentiometer R7A, R7Bwhose wiper is connected through a respective capacitor C7A, C7B topermit variation in the level of the signal coupled between preamplifierand amplifier stages within amplifiers 15A, 15B per se, as isconventional.

Each of amplifiers 15A, 158 has a differential output connected to therespective primary winding T1? and T2P of a pair of transformers T1 andT2, each of these primary windings having a center tap to which power isprovided from a battery B1 of suitable voltage upon the closure of aswitch SW2, this voltage also being supplied to respective supplyterminals for amplifiers 15A, 158, as indicated, as well as to a biasingcircuit. The latter biasing circuit includes a pair of resistors R8 andR9. The junction of these two resistors is connected to the anode ofdiode D1 and thus provides a predetermined bias voltage to thenoninverting inputs of amplifiers 15A, 158. The secondary winding T15 oftransformer T1, preferably having the same number of turns as primarywinding TlP, is connected across a potentiometer Rll whose wiper isconnected through a diode D2 to a capacitor C8. A suitable meter M1 isconnected across the capacitor to provide an indication of the voltagethereacross. Connected across the secondary winding T25 of transformerT2, which is an impedance-matching stepdown transformer, are a miniatureloud speaker 17 or other suitable transducer and a jack J1, the latterproviding for connection of a tape recorder, or other means for externalmonitoring, to the dosimeter.

The circuitry thus described is enclosed within a metallic case orenclosure 19, as illustrated in FIG. 2. Case 19 is preferablyapproximately the size of a cigarette package so that it may be readilyheld in the hand or carried in a pocket. For example, according to onepractical embodiment, case 19 had a width of about 2% inches. It isfound that a case of this size is also highly desirable because it doesnot cause disturbance or aberrations in the pickup characteristics ofcoil 11. It has been found that large enclosures sometimes cause wavereflections and offer other disturbances which are detrimental toperformance. Preferably, the corners of case 19 are rounded somewhat toprevent undesirable reflections or other effects on performance ofpickup coil 11. Pickup coil 11 is covered by a translucent plastic cover21 which protrudes from the top of case 19. Cover 21 provides damage andmoisture protection for coil 11 without affecting its pickupcharacteristics.

FIG. 3 shows in detail the preferred arrangement for pickup coil 11. Asillustrated therein, coil 11 is of elongate cylindrical form having arelatively high number of turns of wire which preferably are linearlywound on a nonmagnetic air core coil form 23 so that the turns of wireare spaced uniformly along the length of the coil. In accordance withthis invention, coil 11 is positioned within the case 19 so thatsubstantially exactly half the length of the coil, L/2, protrudes fromcase 19. This provides electromagnetic shielding of half the length ofcoil 11 beginning at one end thereof. By virtue of its elongate form,coil 11 has a length L substantially greater than its diameter D. In avertical plane, i.e., a plane in which lies the longitudinal centralaxis of coil 11, the coil, if unshielded, would have a radiationpattern, and therefore a receiving pattern, which is shaped like afigure 8. The pattern is represented accordingly in FIG. 3 as a dashedline designated 25. However, because half of the length of the coil,i.e., L/2, is electromagnetically shielded by metal case 19, theradiation pattern in a vertical plane comprises only the circular topportion of figure 8 pattern 25. In the horizontal plane, i.e., the planenormal to the longitudinal central axis of coil 11, the pattern iscircular and thus is substantially exactly uniform over 360 of theplane. In the vertical plane, coil 11 is uniformly responsive oversomewhat greater than 180. The effect of this is to provide coil 11 withuniform responsivity to the radiated pulse energy substantially over aspherical angle of somewhat more than 180. As a result of this uniformresponsivity, high accuracy is obtained. A practical example of coil 11which gave satisfac torily uniform and accurate performance had a lengthL of 1% inches and a diameter D of A inch. The coil was constituted by4,400 turns of wire linearly wound on form 23. Such a relatively largenumber of turns provides the coil with high pickup sensitivity.

In general, it is desired that the length of coil 11 be substantiallygreater than the diameter to insure that the radiation pattern in avertical plane is essentially a perfect circle. As will be appreciatedfrom the foregoing, positioning of coil II in the above manner insuresthat there are no exposed null points in the radiation pattern otherthan when coil 11 is aimed directly away from the source of the radiatedenergy, i.e., when the bottom of the dosimeter is toward the source ofenergy.

Means for insuring that the user of the dosimeter will not be exposed toexcessive radiation levels because of inoperation or malfunctioning ofthe dosimeter is provided by the inclusion of a neon bulb NE within coilcover 21. The hollow coil form 23 readily receives bulb NE, asillustrated, so that the bulb is exposed to radiation regardless ofwhether the dosimeter circuitry per se functions or not. Should bulb NEbe exposed to a field density in excess of a predetermined thresholdvalue, e.g., 20-25 mW./cm. causing ionization of the neon gas in thebulb, a glow indication will be provided which is visible through cover21, thus altering the user that he is being exposed to a radiated fielddensity which is unsafe for further exposure. Other gas-filled coldcathode glow-discharge devices may be used in place of neon bulb NE.

In operation, it is assumed that the dosimeter is positioned withrespect to the source or sources of pulsed magnetic energy whoseradiated energy is desired to be monitored so that the source ofradiated energy is within the hemisphere of uniform sensitivityrepresented by the top half of radiation pattern 25. This would occur,for example, if the dosimeter in the form illustrated in FIG. 2 werecarried by a person in a vertical position in his shirt pocket and thesource or sources are any appreciable distance horizontally, i.e., a fewfeet from the user or are somewhat above the user. As is apparent,accurate sensing also results if the dosimeter is oriented so that theplastic cover 21 over coil 11 is pointed in the general direction of thesource of energy. Switch SW1 is positioned so that it is set to asuitable sensitivity range, it being understood that resistors R1, R2and R3 are chosen to provide sensitivity ranges of interest, e.g., l,10, and 100 mW./cm. it is assumed that gain control potentiometer R7Aand meter potentiometer R11 are set to provide meter Ml with ahalf-scale reading of l, 10 and 100 mW./cm. respectively as to the threeranges. Thus, on the 10 mW./cm. scale, a half-scale reading of meter Mlrepresents a predetermined maximum field density of the radiated energywhich is safe for human exposure.

The calibrated apparatus thus properly oriented, coil 11 picks upradiated energy and a voltage is therefore developed thereacross whichvaries according to the field density of radiated pulsed energy to whichthe user is exposed. The signal across coil 11 is applied throughcapacitor C1 to the optional notch filter ll3 which (if present) filtersout any signal energy at the preselected notch frequency determined bythe values of the filter components and thus prevents such energy frombeing supplied to those portions of the circuitry which provideindication of the field density level of radiated energy. The pulsedradiofrequency energy passed through filter 13 is rectified by diode D1and integrated by capacitor C5. The radiofrequency component of thesignal thus applied by diode D1 to capacitor C5 is shunted to thecircuit common COM through capacitor C5. As a result of integration bycapacitor C5, there is developed thereacross a periodic waveform havingan average magnitude which varies as a function of the field density ofthe radiated energy and which has a period which is inverselyproportional to the pulse repetition frequen cy of the radiated energy.This periodic waveform varies substantially according to a sawtooth ortriangular characteristic and is applied to the noninverting input ofamplifier 215A. The amplified sawtooth waveform is supplied to theprimary winding TIP of transformer Tll. The resultant voltage across thesecondary winding T15 is applied through the tap of potentiometer R11through diode D2, which rectifies this voltage and applies it to acapacitor C8. This capacitor serves as a second integrating means whichintegrates the amplified waveform applied thereto to produce a voltagevarying substantially according to the field density of the radiatedenergy. Meter Ml indicates this voltage and thus provides a readilyperceptible indication of the average magnitude of the sawtooth waveformacross capacitor C5. in this way, the meter provides a visual indicationof the field density of the radiated energy.

The sawtooth waveform across capacitor C5 is also applied to thenoninverting input of amplifier 1518 which amplifies the waveform andapplies it to the primary winding T2? of transformer T2. Thesignalacross the secondary winding T28 is supplied to loud speaker 117thus providing an audio signal whose frequency is the same as the pulserepetition frequency of the radiated energy. In this way, a readilyperceptible audio signal is provided for auditory indication of thefield density of the radiated energy. It should be pointed out that thisaudio signal is not provided only when a predetermined maximum fielddensity is picked up by coil 11 but instead is continuously proportionalto the field density.

Neon bulb NE functions to provide an indication if the field density ofthe radiated energy to which the dosimeter, and thereby neon bulb NE, isexposed reaches a predetermined value causing ionization of the neon gasin the bulb. Accordihgly, the neon bulb will fire at this energy level,e.g., -25 mWJcm? as noted previously, and thus will provide a standbywarning which will alert the user of the dosimeter that he is beingexposed to a level of radiation which is unsafe for continued humanexposure. This will occur even if the dosimeter is miscalibrated, isotherwise inoperative or, for example, if it was forgotten to turn onthe dosimeter by closing switch SW2.

As will have been appreciated by those skilled in the art, the dosimeterapparatus of the present invention is operative as a detector for ormonitor of the radiation level of pulsed radio frequency electromagneticradiated energy, including energy at microwave frequencies, suchradiofrequency energy being thermogenic in nature. However, thedosimeter does not respondto other forms of energy, such as gamma rays,ultraviolet radiation, visible light, or infrared radiation. Sincepickup coil lll does not provide a tuned input and therefore does nothave a high Q" ordinarily associated with a tuned circuit, the apparatusis not responsive to merely a distinct band or bands of frequencies, butit is instead responsive to radio frequency energy over a wide frequencyspectrum. Further, the dosimeter is small in size and quite simple indesignand is therefore inexpensively constructed.

In view of the above it will be seen that the several objects of theinvention are achieved and other advantageous results attainedf Asvarious changes could be made in the above constructions withoutdeparting from the scope of the invention, it is intended that allmatter contained in the above description or shown in the accompanyingdrawings shall be interpreted as illustrative and not in a limitingsense.

What is claimed is:

l. Dosimeter apparatus for monitoring the field density of radiatedpulsed electromagnetic energy comprising:

a pickup responsive to said radiated energy, and constituted by anelongate cylindrical coil having a plurality of turns of wire spaceduniformly along the length thereof;

means for electromagnetically shielding approximately half said lengthof the coil beginning at one end thereof, whereby said coil is uniformlyresponsive to the radiated energy substantially over a spherical angleof at least means interconnected with said coil in an untuned circuitfor producing a periodic waveform having an average magnitude whichvaries as a function of the field density of said radiated energy andhaving a period which is inversely proportional to the pulse repetitionfrequency thereof;

means responsive to said periodic waveform for providing a readilyperceptible indication of the average magnitude of said periodicwaveform whereby the field density of said radiated energy, and therebythe relative human ther'mogenic dosage thereof, is readily monitored.

2. Dosimeter apparatus as set forth in claim 1 wherein said means forelectromagnetically shielding a portion of said coil comprises ametallic enclosure for said apparatus and substantially exactly half thelength of said coil protrudes from said enclosure.

3. Dosimeter apparatus as set forth in claim 1 wherein said means forproducing a periodic waveform comprises means for integrating saidradiated energy whereby said periodic waveform varies substantiallyaccording to a sawtooth characteristic.

4. Dosimeter apparatus as set forth in claim 3 wherein said meansresponsive to said periodic signal comprises a first linear differentialamplifier for amplifying said periodic waveform and means forintegrating said amplified waveform to produce a voltage varyingsubstantially according to the field density of said radiated energy.

5. Dosimeter apparatus as set forth in claim 4 further comprisinga'meter for indicating said voltage to provide a visual indication ofthe field density of said radiated energy.

6. Dosimeter apparatus as set forth in claim 5 wherein said meter iscalibrated to a predetermined maximum field density of said radiatedenergy which is safe for human exposure.

7. Dosimeter apparatus as set forth in claim 3 wherein said meansresponsive to said periodic waveform comprises a second lineardifferential amplifier for amplifying said periodic waveform to producean audio signal providing for auditory indication of the field densityof said radiated energy.

8. Dosimeter apparatus as set forth in claim 7 further comprising atransducer interconnected with said second amplifier and giving auditoryindication of the field density of said radiated energy.

9. Dosimeter apparatus as set forth in claim 2 further comprising agas-filled cold cathode glow-discharge device prising a notch filterinterconnection said pickup and said means for producing a periodicsignal, said filter being adapted to prevent electromagnetic fieldssubstantially at a preselected frequency picked up by said pickup frombeing supplied to the last said means.

1. Dosimeter apparatus for monitoring the field density of radiatedpulsed electromagnetic energy comprising: a pickup responsive to saidradiated energy, and constituted by an elongate cylindrical coil havinga plurality of turns of wire spaced uniformly along the length thereof;means for electromagnetically shielding approximately half said lengthof the coil beginning at one end thereof, whereby said coil is uniformlyresponsive to the radiated energy substantially over a spherical angleof at least 180*; means interconnected with said coil in an untunedcircuit for producing a periodic waveform having an average magnitudewhich varies as a function of the field density of said radiated energyand having a period which is inversely proportional to the pulserepetition frequency thereof; means responsive to said periodic waveformfor providing a readily perceptible indication of the average magnitudeof said periodic waveform whereby the field density of said radiatedenergy, and thereby the relative human thermogenic dosage thereof, isreadily monitored.
 2. Dosimeter apparatus as set forth in claim 1wherein said means for electromagnetically shielding a portion of saidcoil comprises a metallic enclosure for said apparatus and substantiallyexactly half the length of said coil protrudes from said enclosure. 3.Dosimeter apparatus as set forth in claim 1 wherein said means forproducing a periodic waveform comprises means for integrating saidradiated energy whereby said periodic waveform varies substantiallyaccording to a sawtooth characteristic.
 4. Dosimeter apparatus as setforth in claim 3 wherein said means responsive to said periodic signalcomprises a first linear differential amplifier for amplifying saidperiodic waveform and means for integrating said amplified waveform toproduce a voltage varying substantially according to the field densityof said radiated energy.
 5. Dosimeter apparatus as set forth in claim 4further comprising a meter for indicating said voltage to provide avisual indication of the field density of said radiated energy. 6.Dosimeter apparatus as set forth in claim 5 wherein said meter iscalibrated to a predetermined maximum field density of said radiatedenergy which is safe for human exposure.
 7. Dosimeter apparatus as setforth in claim 3 wherein said means responsive to said periodic waveformcomprises a second linear differential amplifier for amplifying saidperiodic waveform to produce an audio signal providing for auditoryindication of the field density of said radiated energy.
 8. Dosimeterapparatus as set forth in claim 7 further comprising a transducerinterconnected with said second amplifier and giving auditory indicationof the field density of said radiated energy.
 9. Dosimeter apparatus asset forth in claim 2 further comprising a gas-filled cold cathodeglow-discharge device protruding from said enclosure for providing aglow indication when said field density reaches a predetermined valuecausing ionization of the gas in said device.
 10. Dosimeter apparatus asset forth in claim 9 wherein said glow-discharge device comprises a neonbulb.
 11. Dosimeter apparatus as set forth in claim 1 further comprisinga notch filter interconnection said pickup and said means for producinga periodic signal, said filter being adapted to prevent electromagneticfields substantially at a preselected frequency picked up by said pickupfrom being supplied to the last said means.